New techniques and materials with cementitious base have been developed recently in the building industry. Important among these is the material known as glass fiber reinforced concrete, sometimes referred to as GFRC. Glass fibers, often used to reinforce plastics are acceptable to reinforce concrete, but experience demonstrated that the conventional glass fibers were subject to alkaline attack, which not only destroyed the bond between portland cement and glass fibers but also structurally weakened the glass fibers.
Alkali resistant glass fibers were developed in the late 1960s and polymers were used in the 1970s to reduce the alkaline attack on the glass fibers. These developments led to the development of glass fiber reinforced concrete panels for building curtain wall applications, i.e., where panels fastened to on the building's structural framework to form an exterior cladding of the structure. Due to the availability of thinner GFRC wall in GFRC panels which results in lighter weight panels, they have become attractive for fabricating cladding on high rise office buildings.
An excellent article on GFRC panels was published by the Prestress Concrete Institute entitled "Recommended Practice for Glass Fiber Concrete Panels." It was prepared by the PCI Committee on Glass Fiber Reinforced Concrete Panels, and published in the Journal of the Prestress Concrete Institute, Vol. 26, No. 1, Jan-Feb, 1981.
Portland cement composites used to form GFRC panels typically consist of about 5% by weight of alkali resistance glass fiber combined with portland cement and aggregates, such as sand, and may include additives. Trained operators can spray up the GFRC panel by directing a hetrogenous spray of cement slurry, aggregate, and chopped glass fibers into a generally horizontal mold of the desired shape, size, etc. The chopped glass fiber will be randomly distributed in the resulting matrix formed in the mold.
Because of the strength of the above matrix, GFRC panels can have nominal wall thickness from 3/8 inch (9.5 mm) to 1 inch (25.4 mm) and a large range in size and shape, which is compatible with the mold and release therefrom. Curtain walls, sometimes referred to as exterior cladding panels, when made with GFRC will have only 1/10 to 1/3 the weight of comparable precast concrete panels. Further, the GFRC panels can be stiffened by forming integral ribs in the panels by laying longitudinal styrofoam battens on the freshly cast panel and overspraying the battens with 3/8 to 1/2 inch of the mixture, thereby encasing the batten. Cladding units generically include wall panels, window wall units, spandrels, mullions and column covers.
In the past, such cladding units have included inserts embedded in the panel for attaching them to the framework to which they are fastened like a picture frame. All insert devices incorporated into the panel must be corrosion resistant. Zinc or cadmium coated ductile steel is satisfactory but consideration must be given to the thermal compatibility of the insert if the GFRC panel is fire rated. Improper embedment of the inserts will lead to serious failures; further, care must be taken to have the insert extend above the panel so any bolting against the insert and the attaching structure will not place the portion of the panel covering the insert under substantial stress, since this will cause a failure.
The attachment of the inserts of the panel to the supporting structure must also allow for dimensional movement of the panel caused by creep, thermal movement and structure tolerances. Thus, slotted and/or oversized apertures in the inserts are employed to suspend the individual GFRC panels in spaced relationship to the adjacent panels, i.e., leaving gaps between their edges. Thereafter, the longitudinal spaces or gaps between the panels are sealed with a mechanical packing and/or sealant. It is recommended that the joints be packed with fiber felt followed with a flexible polyethylene back up rod and then faced on the outside surface with a sealant.
While the above prior art GFRC panels have found acceptance and filled certain architectural requirements, the techniques are costly and restricted to the more expensive type of high rise construction with heavy steel framework to carry the load induced by the weight of the panels.
The current invention involves a type of GFRC panel configuration which eliminates the need for embedded inserts, allowing the novel panels to be bolted together in a building wall structure wherein the panels are self-supporting vertically, with only lateral support provied by a contiguous light weight frame structure. This invention is a substantial departure from the existing state of the art of GFRC panels and is achieved by special panel design, construction and a novel joint arrangements, along with a unique fastening arrangement to the contiguous building framework whereby only lateral loading is transmitted to the framework.
It is an object of this invention to take advantage of the high compressive strength of GFRC panels by a design which creates a selfload bearing wall composed of a plurality of GFRC panels bolted together, as well as a method to create one or more than one such walls to form a building structure.
Beyond the above advantages, this invention provides the elements and method whereby lower cost, more energy efficient and architecturally aesthetic buildings can be constructed.